U.S. Pat. Nos. 4,393,199 and 4,483,978, the teachings of which are incorporated herein by reference, are directed to a method of cationic polymerization of cyclic ethers in which a polyhydric alcohol, e.g., a diol, is mixed with a cyclic ether monomer(s) and an acid catalyst. Polyethers formed from oxetane and tetrahydrofuran (THF) monomers are also described, for example, in U.S. Pat. Nos. 4,405,762 and 4,707,540, the teachings of which are incorporated herein by reference.
Polymers produced by cationic polymerization of cyclic ethers are useful for forming cross-linked elastomers. Cross-linked elastomers are useful, for example, as elastomeric binders for high-energy compositions, such as propellants, gasifiers, explosives, or the like. Such high-energy compositions contain a cross-linked elastomer, solid particulates, such as fuel particulates and oxidizer particulates, and may also contain a plasticizer for the elastomer.
Above-referenced U.S. Pat. No. 4,393,199 describes a synthesis of polyoxetane or polyoxetane/tetrahydrofuran polymers by a cationic, living polymer process. Briefly, a polyfunctional alcohol is reacted with an acid, e.g., a Lewis acid, such as boron trifluoride, to form an adduct. This adduct reacts with a cyclic ether, such as an oxetane or tetrahydrofuran (THF), activating the cyclic ether towards attack by an initiator alcohol or polymer terminal alcohol. The terminal end of the cyclic ether residue is an alcohol and, in a similar manner, attaches to a further activated cyclic ether molecule, opening the ring in the process.
Generally, the functionality, particularly hydroxyl functionality, of the polymer which is produced corresponds to the hydroxyl functionality of the alcohol. Thus, if the alcohol is a diol, such as butanediol, the polyether which is produced has an hydroxyl functionality of about 2. If the alcohol is a triol, the polymer has a hydroxyl functionality of about 3. Side reactions or incomplete initiation from all hydroxyl groups may result in a polymer which varies slightly from the functionality of the alcohol precursor.
Cyclic ethers formed from oxetanes or oxetanes plus THF have important potential as binders for high-energy compositions, such as propellants, explosives, gasifiers, or the like. Cured polymers formed from oxetanes and/or THF are elastomeric material and are capable of carrying high levels of particulate materials, such as fuel particulates and/or oxidizer particulates. Depending upon the oxetanes which are used to form the polymers, the high-energy plasticizers may be compatible with high levels of energetic plasticizers, e.g., nitrate ester plasticizers. Most commonly, oxetane and oxetane/THF polymers which have been used to form elastomeric binders difunctional, e.g., having a pair of terminal hydroxyl groups. Such difunctional polymers must be cured with a curing agent, e.g., an isocyanate of functionality substantially higher than two. For example, a mixed isocyanate curative sold under the tradename Desmodur N-100.RTM., having a functionality of about 3.6, is often used to cure difunctional oxetane and difunctional oxetane/THF polymers.
A problem with elastomeric binders formed from oxetane and oxetane/THF polymers is their tendency to have mechanical characteristics less than that which would be desired for a high-energy composition, particularly for a rocket motor propellant. It is especially difficult to provide binders formed from oxetane and oxetane/THF polymers having adequate stress capabilities. Recently, it has been found that better stress capabilities are achieved using trifunctional polymers formed from oxetanes and oxetanes plus THF. It has been considered that even better stress capabilities might be achieved using tetrafunctional polymers; however, successful polymerization of oxetane and oxetane-plus-THF to produce high purity tetrafunctional polymers has not been achieved.
U.S. Pat. No. 4,393,199 suggests polymerization using tetrafunctional alcohol molecules, particularly pentaerythritol. Presumably, a living polymer grown from each hydroxyl group of the pentaerythritol molecule would produce a tetrafunctional polymer. However, it is believed that successful cationic polymerization from pentaerythritol has not been achieved. This is not to say that no molecules of tetrafunctional polymer have been produced by cationic polymerization from pentaerythritol, but that a very substantial proportion of the polymer molecules produced have functionalities less than four. A major problem with using pentaerythritol is its relatively polar nature. For cationic polymerizations, oxygen-containing solvents are generally excluded, requiring the use of substantially non-polar solvents in which pentaerythritol is substantially immiscible. It is further believed that the close proximity of the four hydroxyl groups of pentaerythritol molecule often results in polymer chain initiation from less than all of the four hydroxyl groups.
It is a general object of the present invention to provide tetrafunctional polymers formed from oxetanes and/or tetrahydrofuran.